Thermal spray stripping reasons

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In the ideal world, thermal spray coatings shops are to apply coatings once on to the surface of the workpiece, ship it to the customer, bill them and get paid with both the customer and the supplier in a state of happiness. In such a world, the question of stripping and recoating thermal spray coatings does not exist. However, in the real world, that is not always the case. There are numerous instances when a thermal sprayed surface needs to be stripped and the part re-coated. It is the purpose of this post to identify the reasons behind stripping and recoating thermal sprayed parts. The order in which the reasons are listed do not conform to any heirarchy. Additionally, please note that we will be dealing only with stripping and re-coating thermal sprayed coatings on actual hardware. That is, we will not deal with the stripping of hard tooling used in the thermal spray processes. As you may know, often hard tooling maintenance involves stripping the tooling of any overspray catches and bringing back the tooling to its original state so that proper masking and shadowing is still accomplished.

Reason number one: The wrong surface got flame sprayed While some thermal spray coatings shop owners may not admit to it, this is a problem that I have seen happen more than once in my career. Sometimes the print was read incorrectly and the wrong instructions were sent to the shop floor. This is different from the case wherein the right instructions were sent but the masking, grit blasting operators etc read the instructions incorrectly. Many times this latter situation gets rectified if you have an in-process inspection in place that verifies if the masked area is exactly per what the operation sheet calls for. Well, if the engineering instruction itself was incorrect, you have no choice but to strip and re-thermal spray the right area.

Reason number two: The wrong thermal spray coating material was used. This has happened in my career too. In a real life example, the print called for a twelve percent yttria stabilized zirconia, but the part ended up getting a plasma spray coating of eight percent yttria stabilized zirconia. The customer was contacted to see if they would accept the part on a deviation request since they were both thermal barrier coatings, but the customer said no. And so stripping and recoating was unavoidable. Such errors can be minimized if the powder control room technician is over careful. Companies where this has not happened should really pat such technicians on the back – they are usually the unsung heroes.

Reason number three: The incorrect process was used. This can happen in thermal spray coatings houses that perform a variety of processes. And this is more apt to happen with certain coatings that are coatable by different processes. An example will make this clear. Tungsten carbide cobalt aggregates can be sprayed by HVOF processes as well as plasma coating processes. The same is the case with cobalt-chrome-molybdenum-silicon powder such as the well known T-800. The chance of this happening with alumina-titania is virtually non-existent since it is generally coated only by plasma. And as one may know, T-800 coated by HVOF is much different from T-800 coated by plasma. And in such a case, one has no choice but to strip and re-coat.

Reason number four: The thermal sprayed coating does not meet the customer’s metallurgical quality specifications. This encompasses the entire range of metallurgical specifications including but not limited to porosity, grit blast interface contamination, micro-hardness, macro-hardness and tensile bond strength requirements. These can be minimized ( but not completely eliminated ) by a rigorous thermal spray booth release procedure. This will ensure that the booth is operating in the best operable condition for the coating at hand. This does not ensure that every single part in the batch will meet the requirements. For example, the booth release coupon may have met all of the metallurgical lab requirements, however, half way through the coating process, the powder feeder noticeably started malfunctioning with tremendous surges in feed rate resulted in potentially high levels of unmelted particles, etc resulting in the coupon coated with the part failing quality. In this case, the part may have to be stripped and re-coated.

Reason number five: Thermal spray coating thickness violates customer specifications. This may have been determined with simulated part samples that are coated with the part or by non-destructive fisher scope readings or even standard caliper readings. If the coating thickness is above max or non-uniform, then the part becomes a candidate for strip and re-coat.

Reason number six: There are cases by design where thermal spray strip and re-coat are required. An example is the case of overhaul and repair components in the aircraft engine industry. In the engine overhaul and repair industry, the FAA repair manuals may call for an engine run component that has had thermal spray coatings to be stripped of them and re-coated as part of the refurbishment process. This is a lucrative industry by the way for those in the thermal spray industry that have a great quality system in place and have oem approvals by the major players in the industry.

As you can see above, there are at least half a dozen reasons why thermal spray stripping and re-coating is a possibility in the every day life of a thermal spray business. In a future post, we will discuss more about the stripping processes themselves and the things to be careful about and watch out for.